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Reverse Osmosis Remineralization

I can make this for pennys on the dollar compared to commercial premixed compounds
costing a magnitude higher. Additionally, I've removed from the mix all of the
Sodium Chloride/Sulfate and Potassium Sulfate compounds included in the commercial versions, which I think
are unnecessary/unwanted and added only to increase the weight of the final
product (fillers).

H2CO3 is carbonic acid. Sodium bicarbonate is NaHCO3. The important part of this is the HCO-3 radical which is part of the bicarbonate cycle which will assist with buffering your system. You will still have sodium chloride in the system as the sodium bicarbonate will dissociate into sodium ions and hydrogen carbonate ions and there are chloride ions from the dissociation of calcium and magnesium chlorides, however, as you say, they are not added as extras or fillers. What volume of water are you using? I don't find using the 'parts' unit particularly useful. Are you adding solids or solutions? How big is the 'part'? A better unit is mg/L. For example, the chloride ions concentration for aquatic organisms should not exceed 600mg/L. You would want to make sure you did not exceed this. Drinking water should be no more than 250mg/L. You have two different sources of chloride ions in the calcium and magnesium chloride. This is useful if making your own mixes because it gives you a specific weight:volume ratio for your mixes. To calculate the chloride component you need to understand that the molecular mass of calcium carbonate is 110.98 g/mol, the atomic mass of chlorine is 35.453g/mol, and the atomic mass of calcium is 40.078g/mol (40.078 +2(35.453)=110.98g/mol) = 36.12% calcium and 63.88% chlorine (as chloride ions). Therefore, 70.906g per 110.98g (or 63.88g per 100g) of calcium chloride will be chlorine as chloride ions. 1g=1000mg. 100g of calcium chloride in one litre of water = 63.88g/L=63880mg/L chloride conc. To achieve drinking water levels you would want no more than about 0.3g/L of calcium clhoride (not taking into account the magnesium chloride). The same process can be worked out for magnesium chloride.

The mix of components would be the same for any given volume of water. "Parts" could be replaced by grams, ounces, pounds, etc., depending on how much a person wished to premix. It is only a value of relation between the chemicals.

How much of the mix to use for a given volume of water would depend on the TDS output of your RO system (not all of them will not
get you to a perfect zero) and the desired total TDS value once remineralized.
You would want to add just enough to hit your target total as measured by a
digital TDS meter. Only add a little bit at a time and check with the meter.
Keep track of how much of the mix you add to a given volume of RO water and that
will be how much need to use every time you make a batch of water. It will not
be a static value however, because RO output TDS will increase as the membrane
ages so it will need to be rechecked occasionally, maybe every couple of months
or so depending on how much water you move through the membrane. When you determine how much of the mix is needed to reach a desired TDS value for a given volume of water, you can convert to mg/l for simplification.

Thanks for the feedback! The formulation I use has been working great for me and gets me to the desired TDS target consistently.

I can make this for pennys on the dollar compared to commercial premixed compounds
costing a magnitude higher. Additionally, I've removed from the mix all of the
Sodium Chloride/Sulfate and Potassium Sulfate compounds included in the commercial versions, which I think
are unnecessary/unwanted and added only to increase the weight of the final
product (fillers).

Ok... that makes sense now. You are adding the minerals until the TDS reaches your target and not trying to make a pre-determined mix that will hit the mark. That's what I do when making artificial seawater for my reef tank, too, based on salinity instead of conductivity/TDS.

As an interesting aside... the inclusion of the sodium bicarbonate is interesting because it will help to buffer your tank against rapid pH swings. Carbon dioxide will enter the water via the water surface and from the respiration of the fish and other aquatic organisms and will dissolve to form carbonic acid (I reckon this is where you got the carbonic acid from and you actually knew sodium bicarbonate was NaHCO3 but I reckon you were probably researching the bicarbonate cycle and saw the carbonic acid reference and put that down when you meant something else.. isn't it great when our brains work faster than our fingers LOL). The Carbonic acid will dissociate in water to form bicarbonate ions (hydrogen carbonate ions) and hydrogen ions. This occurs in three distinct steps which are each equilibrium reactions.

So CO2 + H20 <->H2CO3 <-> H+ + HCO3-

So by adding sodium bicarbonate, which dissociates into sodium ions (Na+ and bicarbonate ions (HCO3-) you will effectively be pushing the reaction to the left to form more carbonic acid which in turn will push further to the left to make more CO2 and water... which in turn will push back the other way and kick start the bicarbonate cycle which helps to buffer the whole system... Adding a bag of crushed oyster shells will do a similar thing. We use to add a bag of crushed oyster shells (or marble chips) to a filter in a bag to provide the carbonate ions to also kick-start the whole thing. Aint chemistry awesome!

I have always attempted to explain how maintaining an aquarium aka micro ecosystem is a science experiment. I have known a few kids who I helped start their own aquarium and wrote science papers on their experience. Those papers dived into the nitrogen cycle as well as water chemistry. The deeper you dive into understanding the more complicated it can become. As they say the more you know the more you realize you know nothing.

With my own water (tap) I have to deal with 2 sources that change in the late spring and early fall. And, until I got a handle on this change I would lose fish every time. Spring water averages pH: 6.4-6.8, KH: 0-3, GH: 3-5 and I now add buffers to stabilize the pH; and the Summer water comes from the river and is very different with pH: 7.0-7.4, KH: 3-6, GH: 6-8. As I tend to do large water changes I would end up killing my fish from a simple water change. Fish struggle, but are able to survive when the pH jumps up; but drop like flies when it goes the other way.